In commercial aircraft having jet engines, it is oftentimes necessary to perform maintenance and repair work on the engine during stopovers along the flight route. Most jet engine cowlings are constructed as two bifurcated half cylinders hingedly attached to the engine or mounting strut so that they can be pivoted upwardly away from the engine core to permit maintenance personnel to access the engine core. Present devices used for opening the cowl halves or "C-ducts," as they are known in the industry, include mechanical screw devices which have poor reliability, hydraulic actuators which are operated either with a separate hand pump, or a powered cowl door opening system such as that disclosed in U.S. Pat. No. 4,399,966.
As is the case with all "dead ended" hydraulic systems in aircraft, including that which is disclosed in U.S. Pat. No. 4,399,966, structural deflections and temperature changes in flight produce significant stress cycling on hydraulic system components. A high pressure relief valve in fluid communication with the hydraulic system will prevent the system from exceeding limit loads because the valve limits the peak pressure. However, variations in the hydraulic fluid pressure below the peak pressure will result in the generation of cyclic loads on the cowl structure, such as actuator fittings, and eventually will cause fatigue failure of that structure.
It is, therefore, an object of the present invention to provide an apparatus that will normally vent the actuator directly to the system return, thereby opening the hydraulic system and defining a very low pressure relief valve.
It is another object of this invention to provide the same relief valve with the capability of being selectively energized, thereby closing the hydraulic system and defining a high pressure relief valve that will provide relief to the system return if a predetermined pressure limit is exceeded.
It is yet another object of the present invention to provide the valve with a manual override such that the valve may be manually or mechanically set into the closed position, and therefore, into the high pressure relief mode.